This invention relates generally to multi-color displays and more particularly to light sources useful in liquid crystal displays (LCDs).
As is known in the art, multi-color liquid crystal displays have a wide range of applications. One such display includes a liquid crystal display panel made up of a plurality of liquid crystal switching devices arranged in a matrix of rows and columns. Each set of 3 (or 4) adjacent switching devices makes up a picture element, or "pixel", for the display panel. Each one of the switching devices in each set thereof is formed with a semi-transparent filter disposed thereon having a different one of 3 (or 4) of the primary colors (red, blue, or green). Behind the panel is a white light source. The optical transmittance or non-transmittance of each one of the switching devices in the matrix is controlled by a voltage applied between opposite surfaces of the liquid crystal switching device. The applied voltage controls the rotation of the plane of polarization of the incident light and thereby the fractional portion of the incident light transmitted through a second polarizer (i.e. the analyzer). Thus, in response to one voltage level, the plane of polarization of the light exiting the liquid crystal switching device is aligned with that of the second polarizer so that the incident light passes through the second polarizer (albeit with the color of the filter disposed on the activated switching device) while in response to another voltage level, the plane of polarization is orthogonal to that of the second polarizer so that ideally none of the incident light passes through the switching device. In any event, it should be noted that much of the incident light may be lost even when the switching device is activated to transmit incident light. Further, in the three color case, each filter will allow, to a first approximation, only one-third of the incident light to pass through it. In practice, because of this and other effects, only about 21/2 of the light incident on the first polarizer is completely transmitted through the succession of first polarzer, switching device, filter, and second polarizer (analyzer).
For viewability, in sunlight illumination, a typical display might require 200 fL of luminance output. To achieve this would generally require 8000 fL luminance of the light source, i.e. a light source comparable to the brightness of a snow field in full noonday sunlight. Such a combination of a multi-color liquid crystal display (LCD) and an 8000 fL light source borders on impracticality because of (a) the temperature rise due to light energy absorbed in the filters and polarizer (b) the power requirement of the light source and (c) the problem of dissipating heat generated by the light source. While the use of a field emission cathode array with a white phosphor screen has been suggested as an alternative white light source, the power required would still be significant.
An alternative to the above described white light source multi-color LCD display is the use of a plurality of switching devices again arranged in a matrix of rows and columns. Here, however, a single liquid crystal switching device is used for each pixel of the display. Furthermore, each of the switching devices is a monochrome device; that is, it does not- have a color filter. The array, however, is back lit with a light source adapted to produce light of the three primary colors. Conceptually, the back of the LCD panel would be sequentially illuminated in red, then blue, and then green, for example. Prior to each color illumination (i.e. color field), the switching devices in the LCD panel are addressed row by row and when addressed, each switching device in such row is set in either the transmittant, non-transmittant, or some transmissive level or state in between, in accordance with the information to be displayed. However, because of the time required for the switching device to change transmittance level or state, one must wait for the switching devices in the last row in the array to change state before the back is illuminated with a different color. Time delay must also be provided to account for light decay of the light source. Unfortunately, the time delays in this conceptual system are of such magnitude as to prevent its practical implementation. One technique suggested is to sweep a first one of three primary colors from a first edge of the display to the opposite edge of the display and then sweep a second color from the first edge to the opposite edge, the second color sweep being initiated prior to the termination of the sweep of the first color. Next, a third color is swept from the first edge to the opposite edge, such third color sweep being initiated prior to the termination of the second color sweep. Thus, the three colors appear to pass as waves from the first edge of the display to the opposite edge of the display. Some time is allowed between the termination of one color on a given row and the excitation of the next color, to allow for the finite time taken by the switching devices to change transmissivity in response to addressing signals.